1. Field of the Invention
The present invention relates to a device for checking mobile electrical charges in a MOS integrated circuit.
2. Discussion of Background
The elements of integrated circuits, such as MOS transistor grids are formed by superimposing three layers, namely a metal layer M forming the grid electrode, an oxide layer 0 and a layer S constituting the doped silicon wafer. The oxide layer 0 contains the mobile ionized impurities which, trapped in the oxide, can migrate and behave in the same way as mobile electrical charges. As a function of the potential difference applied between the metal electrode and the semiconductor layer, they move towards the metal-oxide boundary or the oxide-semiconductor boundary.
As is known, a three-stage process is used for revealing the mobile electrical charges. In a first stage, for each of the points of the integrated circuit which it is wished to test, a capacitance-electrical voltage characteristic C=f.sub.1 (V) is recorded, in which V is the potential difference applied between the surface of the integrated circuit and the semiconductor layer S and C is the capacitance of the oxide layer 0 and the semiconductor to the right of the tested point. To this integrated circuit point is then applied a voltage-temperature stress. The effect of the temperature stress is to activate the diffusion of the mobile ionized impurities of the oxide layer, whilst the effect of the voltage stress is to orient the displacement of the mobile ionized impurities activated in this way. The thermal stress is approximately 200.degree. C. and the voltage stress, proportional to the oxide layer thickness, is approximately a few volts for an oxide thickness of 50 nm. In a third stage and after the return to ambient temperature, a further capacitance-voltage characteristic C=f.sub.2 (V) of the integrated circuit point is recorded. This second characteristic differs from the first. In addition, the theoretical capacitance - voltage characteristic C=f.sub.1 (V) is known. The value of the capacitance C.sub.bp corresponding to V=0 on this characteristic is called the "flat band capacitance".
In order to evaluate the number of mobile electrical charges, on each of the experimental characteristics, the flat band voltages V.sub.1 and V.sub.2 are recorded and are respectively given by the equations C.sub.bp =f.sub.1 (V.sub.1) and C.sub.bp =f.sub.2 (V.sub.2). The comparison between V.sub.1 and V.sub.2 gives an indication of the mobile electrical charges present in the oxide layer to the right of the tested point.
In order to realize a known checking or testing process, the expert uses a device which mainly incorporates a slab or plate support provided with heating means, on which is deposited a semiconductor plate or slab containing the integrated circuits to be tested and the electrical contact means formed by electrical points for electrically exciting the points of the integrated circuits to be tested. The known devices have several contact means. Thus, it is possible to record the number of mobile charges in the oxide at several points on an integrated circuit wafer. In general terms, the first stage of recording a capacitance-voltage characteristic is sequentially carried out for each point to be tested. This is followed by the second stage of the temperature-voltage stressing and is carried out simultaneously for the same points. This is followed by the third stage of recording a further capacitance-voltage characteristic and this is carried out sequentially for each of these points.
The main disadvantage of this device is that it only makes it possible to test, in practice, a few points on the integrated circuit wafer. Thus, the temperature-voltage stressing takes a long time, i.e. approximately 10 minutes. If it is wished to test more points of the integrated circuit board than there are electrical contact means, it is necessary to carry out a number of individual operations, involving in each case the temperature-voltage stressing of as many points to be tested as there are electrical contact means. Thus, the more points which have to be tested, the more time-consuming the operation. Thus, in practice, the expert limits the number of tested points to the number of available electric contact means, i.e. to a few units.